1. Field of the Invention
The present invention relates to a multi-beam scanning device, which performs optical image recording using plural light beams and which is for use in image forming apparatus such as laser printers, digital copiers, facsimile machines, etc. In addition, the present invention relates to an image forming apparatus using the multi-beam scanning device.
2. Discussion of the Background
In recent years, a need exists for image forming apparatus such as laser printers, facsimile devices and digital copiers, which can perform high speed and high density recording. In this case, if high speed recording is performed using a single beam scanning device, it is necessary to use a deflector which can rotate at a high speed. Such a deflector is expensive and in addition causes wind noises due to the high speed rotation. Therefore, it is necessary to provide a soundproof device, resulting in further increase of costs. In order to solve such a problem, a multi-beam scanning device which irradiates an image bearing member such as photoreceptors with plural laser beams at the same time has been used. By using such a multi-beam scanning device, high speed and high density recording can be performed without rotating a deflector at a high speed.
It is necessary for such a multi-beam scanning device to scan plural laser beams at a desired beam pitch (i.e., interval between two adjacent scanning lines) in view of image quality. In particular, recently a need for high density images increases more and more to produce high quality images such that the scanning line density is 1200 dpi (dot per inch).
In order to perform high density recording, a method in which plural light sources and plural coupling lens are rotated around an axis which is perpendicular to the main scanning direction and sub-scanning direction is known. Published unexamined Japanese Patent Application No. 2001-13432 discloses a technique such that only plural light sources and plural coupling lens are rotated while the aperture configured to perform beam regulation is not rotated around the rotation axis. By using this technique, change of beam pitch caused by changing and adjusting operations of the beam pitch can be prevented and thereby a desired beam pitch can be obtained.
In such a method, the beam pitch adjusting operation has to be performed while the scanning device is attached to a housing of the optical unit. However, the positions of the light sources relative to the coupling lens have been adjusted. If one or more light sources are damaged and thereby image qualities seriously deteriorate, there are two methods for repairing the scanning device. One of the methods is to exchange the unit holder including the light sources and the coupling lens, followed by adjustment of the beam pitch. The other of the methods is to exchange the multi-beam scanning device itself, inclusive of the housing, which device has been subjected to beam pitch adjustment. It takes a long time to perform such an exchanging operation and beam adjustment operation. In addition, when performing such operations, it is necessary to remove parts and units associated to the scanning device. Therefore, service engineers typically dislike such operations. In view of such circumstances, a need exists for a multi-beam scanning device which service engineers can easily exchange and which can be used soon after being set to an image forming apparatus without performing an adjustment operation. In addition, there is a need to visually confirm whether the beam pitch of the exchanged scanning device is proper.